Winterspring is one of the best zones for classic thorium farming. It has one of the highest amount of Rich Thorium veins in the game. While that leads to a lot of competition, this is also the only place where icecap grows. So if you also have Herbalism, you can double up great on this route.
While Un’Goro Crater has less Rich Thorium veins, it can still be great, even though you will get less Arcane Crystals here. There is less competition here, so it can be worth it. You will also pick up some Mithril here, as this zone is not as high a level as the others.
The Burning Steppes are amazing if you have Mining and Herbalism, as you can pick up a bunch of Sungrass, Firebloom and some Dreamfoil on your way. But even if you only have Mining, it’s on par with Un’Goro Crater. You could also include the eastern mountain range into the route, but there are less Rich veins there. All in all, the zone is good for classic thorium farming, but also for herbalists.